Granular media filter systems have been employed for many decades and generally provide a bed of granular media, such as sand, through which is passed a particulate - containing liquid inlet stream, the particulates being captured by the sand bed with desired clean liquid effluent exiting from the bed. As the bed becomes increasingly loaded with particulates from the solids-liquid inlet stream, a back pressure builds-up and at a desired point it is necessary in these standard filters to stop the filtration cycle and wash the bed. This is normally done by backwashing the bed (U.S. Pat. No. 3,395,099) or fluidizing the bed. Other systems such as that in U.S. Pat. No. 3,512,640 address the problem of cleaning the bed by recycling upflow liquid before it reaches the top of the bed so that it then flows downwardly from the top of the bed. This counteracts the force of the upward flow of the inflowing liquid. Contaminated bed solids are removed from the bed bottom and new media replaced through a valve at the vessel top. Other systems known commercially as the Dynasand system of the Parkson Corporation and the Strata Sand system of Ashbrook-Simon-Hartley Co., are shown in U.S. Pat. Nos. 4,126,546 and 4,060,484, respectively. The former system involves a method and apparatus where a liquid-solids suspension flows in an upward direction through a filter bed and is discharged from the bed top as filtrated liquid phase. During the filtration the filter medium flows downwardly through the bed zone in counter current flow to the upward suspension. Dirtied media at the bed bottom is transported separate from the bed to a wash path within the overall vessel for washing dirty filter media in counter current flow with a wash liquid along the wash path. The washed filter media is then returned to the top of the filter bed zone. In U.S. Pat. 4,246,102 various flocculation chemicals are added to the feed stream directly prior to feed stream entry into the filter bed. In 4,060,484, there is co-flow of the feed liquid with the downwardly moving bed.
In the related application, a suspension up-flow filter vessel is provided with feed being passed into a filter bed in the filter vessel with clean effluent exiting the vessel top. A side stream of cleaned effluent is piped to the bottom of a separate stand-alone washer. This stream generally moves upwardly and by a co-current vortical washing action, washes dirtied filter media transported from the bottom of the filter bed in the filter vessel to the top of the separate washer. The dirtied filter media cascades down baffles in the interior of the washer. The resultant dirtied cleaning stream normally is pumped to waste from the wash vessel top with a side stream thereof used to transport the dirtied filter media from the bottom of the filter vessel to the top of the separate washer. This transport technique is also seen in South Africa Published Application No. 28544 (1972).
One of the problems in the apparatus of the related application and in the methods and apparatus shown in the '546 and '102 patents, is a tendency of part of the feed stream to be pumped to the separate washer along with the dirtied filter media or into the washer column located centrally of the filter vessel in the latter patents. This tends to increase the amount that passes into the waste stream. The prior art methods further have limited flexibility in controlling the headloss in their filter beds.